The invention set forth herein relates generally to modulating fuel/air controls for gas burners, and more particularly to a system of the type in which heat output of a burner is varied by varying air flow to the burner and in which a substantially constant fuel/air ratio or other fuel/air characteristic is maintained by varying fuel gas flow in response to the air flow.
For a variety reasons in apparatus for heating space and/or hot water for domestic use, it is frequently desirable to employ mechanical means such as a fan or blower for inducing or forcing air flow through a combustion chamber. In such systems, it is also frequently desirable to modulate the heat output of the burner. A common system arrangement for accomplishing these objectives utilizes a variable speed fan or blower under thermostatic control. A signal indicative of the air flow through the combustion chamber is used to modulate the output of a fuel gas regulator valve which supplies gas to the burner. The air flow signal may be a pressure differential generated across an orifice or by means of a venturi section in the air flow passage. An objective of the system design is to for example maintain a substantially constant fuel to air ratio at the burner so as to provide a fuel mixture in which combustion is easily started and high efficiency combustion is maintained.
Apparatus of this general type is disclosed in European Patent Application 0 390 964. More specifically, the differential pressure indicative of air flow is applied across a large area control diaphragm of an amplifying pneumatic control module. Displacement of the large area diaphragm is communicated to a smaller area regulating diaphragm by means of a spring. The regulating diaphragm carries a closure member of a bleed valve which affects a control pressure in a main gas valve containing a secondary servo valve whose closure member is spring biased to limit the maximum output gas pressure. Adjustable biasing of the large and small diaphragms in the control module is accomplished by means of a spring and an associated adjustable retainer screw.
Somewhat similar control apparatus is disclosed in European Patent Application 0 326 880 in which a first diaphragm chamber of a control module is connected to a venturi nozzle in the combustion air passage of a burner system, and an opposing second diaphragm chamber is connected to a pressure port downstream from the venturi nozzle. The control module directly actuates the main gas valve by means of a valve rod connecting the control module diaphragm to the gas valve closure member.
Another burner control arrangement is shown in German utility model publication 83 00 157 in which gas and air flow to a burner are controlled by separate controllers or valves. The air flow controller includes a thermostatically controlled pressure regulator. The regulated output pressure of the air flow controller is supplied to the gas valve through a pneumatic amplifier therein as its control signal.
Although the previously described systems provide modulating operation and are capable of achieving a desired fuel/air ratio for a particular type of gas under relatively constant pressures and other parameters, additional adaptability for use with other types of gasses and under more variable conditions would be desirable. The applicant has devised a burner control system which provides improvements in meeting these objectives.